A CZ (Czochralski) process is used which grows a pillar-shaped crystal from a molten material liquid in a crucible in order to grow a semiconductor single crystal. Usually, in the growth of a semiconductor single crystal, a method of controlling the resistivity of a single crystal to be grown is used. In the CZ process, impurity elements called dopants are added into the molten liquid in the crucible in order to control the resistivity of the grown single crystal. Since, however, the dopants generally have a segregation coefficient not equal to 1, the concentration of the dopants in the crystal changes as the length of the crystal increases in an ordinary CZ process. This is a problem in producing semiconductor single crystals where the resistivity of the crystals is controlled in accordance with the concentration of the dopants.
In order to solve this problem, a continuous charging method or a technique using a double crucible has been proposed where the starting material is continuously fed into a crucible and the concentration of dopants in the molten material liquid is kept constant (published unexamined Japanese patent application 63-79790). As a starting material feeding device used in the continuous charging method, there has been a proposal in which a place for melting the starting material is separated from a place for growing a single crystal to which the molten material is transported (Published unexamined Japanese patent applications Sho 52-58080 and 56-164097). Another proposal is the use of a rod-shaped starting material (published unexamined Japanese patent applications Sho 56-86397 and 62-105992).
The former two continuous charging techniques (that is, described in the applications Sho 52-58080 and 56-164097) require two crucibles, one is for melting the starting material and the other is for growing a crystal. As a result, device structure becomes complicated. In addition, the feeding of the material is difficult to control. The latter two techniques (that is, described in the applications Sho 56-86397 and 62-105992) require a large temperature gradient in each of the crystal growth place and material melting place in the crucible in order to melt the starting material rod with the molten material liquid in the crucible. However, it is very difficult to provide a temperature gradient for melting the starting material during the crystal growth. In addition, the technique described in the application Sho 62-105992 uses a high frequency power to preheat the starting material, which is not practical because, for example, an electric discharge may occur with high probability in a reduced pressure furnace used in the growth of single crystal silicon. In the double crucible technique, since the inner crucible used for the growth of the crystals maintains the molten state of the starting material by the heat radiated from the outer crucible, the heat cannot sufficiently be transmitted from the heater. As a result, a polycrystal is likely to be produced from the inner wall of the inner crucible, resulting in the decrease in the growth rate. In addition, there is another problem that a large quantity of impurities may enter from the crucible material into the starting material.